A swing arm supporting structure for a motorcycle is known wherein a pivot shaft for mounting a rear arm as a swing arm to a rear portion of a vehicle frame is arranged on a side of a drive shaft for transmitting power to a rear wheel. See, for example, JP-A-2002-87364.
FIG. 3 and FIG. 4 in JP-A-2002-87364 will be described below. Reference numerals used hereinafter are the same reference numerals as set forth in JP-A-2002-87364.
FIG. 4 in JP-A-2002-87364 illustrates a state wherein rear arm brackets 7 are mounted to a pair of left and right main frames 6, 6 with a pivot shaft 10 is mounted to left and right frame portions 7e, 7f extending downwardly from upper joint portions 7c of the rear arm brackets 7. A rear arm 14 is swingably mounted to the pivot shaft 10 with a universal joint portion 42 of a drive shaft being provided for transmitting power to a rear wheel. The universal joint portion 42 is arranged in a recess 7b of the rear arm bracket 7, and the universal joint portion 42 and the pivot shaft 10 are arranged in a direction of the width of the vehicle. A foot rest bracket 70 is provided for covering a side of the recess 7b. 
The pivot shaft 10 is formed with a screw portion 10a at one end thereof, a screw portion 10b at the other end thereof. The screw portion 10a is screwed into the frame portion 7e and the screw portion 10b is fixed to the frame portion 7f with a nut 11.
FIG. 3 of 0505-1448PUS1 illustrates a state in which the rear arm brackets 7 are substantially aligned one above another. The universal joint portion 42 connected to a front end of the drive shaft 43 is positioned on the side of the rear arm brackets 7, and shock absorbers 28 are mounted across upper portions of the rear arm brackets 7 with links 22 mounted to lower ends of the rear arm brackets 7.
A swing arm supporting structure for a motorcycle in the related art is known wherein a rear arm is provided for supporting a rear wheel to a rear arm bracket on a vehicle body frame side via a pivot shaft. See, for example, JP-A-2002-87364.
FIG. 4 of JP-A-2002-87364 illustrates a structure in which a pivot shaft 10 is mounted to left and right frame portions 7e, 7f provided on a rear arm bracket 7. A front end portion of a rear arm 14, that is, a cylindrical head portion 14c is supported by the pivot shaft 10.
When the rear arm 14 is mounted to the rear arm bracket 7, the head portion 14c of the rear arm 14 is inserted between the frame portions 7e, 7f of the rear arm bracket 7, and the pivot shaft 10 is inserted into a mounting hole on the frame portion 7f, a pivot shaft insertion hole 14d of the head portion 14c, and a female screw of the frame portion 7e in sequence.
In JP-A-2002-87364, the frame portion 7e of the rear arm bracket 7 is formed with a female screw for receiving the screw portion 10a of the pivot shaft 10 screwed therein. However, since the foot rest bracket 70 is adapted to be detachably attached to the rear arm bracket 7 in order to form the female screw, the foot rest bracket 70 itself and a plurality of bolts are added for mounting the foot rest bracket 70 to the rear arm bracket 7. Thus, the number of components is increased in comparison with the foot rest bracket provided integrally with the rear arm bracket. In addition, the number of process steps for forming a bolt insertion hole for passing or screwing the bolt therein and the female screw is increased.
Therefore, by employing a structure in which a simple bolt insertion hole for passing the screw portion 10a at one end of the pivot shaft 10 is formed on the frame portion 7e from the frame 7f side so that the nut is screwed into the screw portion 10a passed through the bolt insertion hole instead of forming the female screw on the frame portion 7e, the foot rest bracket 70 can be formed integrally with the rear arm bracket 7.
However, since the upper connecting portion 7c of the rear arm bracket 7 is located above the pivot shaft 10, and the shock absorber 28 is located behind the pivot shaft 10 in the vicinity of the rear arm 14 or the rear arm bracket 7. Thus, it is difficult to rotate the nut and screw in the screw portion 10a or to insert a tool for rotating and removing the nut to a position of the nut on the pivot shaft 10 side. In addition, it is further difficult to rotate the nut with the tool.